1. Field of the Invention
This invention relates to electrical switching apparatus and, more particularly, to circuit breakers, such as, for example, arc fault circuit breakers. The invention also relates to a method of operating a circuit breaker. The invention further relates to a trip assembly for a circuit breaker. The invention also relates to a bimetal compensation circuit for a circuit breaker.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
Subminiature circuit breakers are used, for example, in aircraft electrical systems where they not only provide overcurrent protection but also serve as switches for turning equipment on and off. A circuit breaker push-pull handle is moved from in-to-out in order to open the load circuit. This action may be either manual or, else, automatic in the event of an overload or fault condition. If the push-pull handle is moved from out-to-in, then the load circuit is re-energized. If the load circuit had been automatically de-energized, then the out-to-in operation of the push-pull handle corresponds to a circuit breaker reset action.
Typically, subminiature circuit breakers have only provided protection against persistent overcurrents implemented by a latch triggered by a bimetal responsive to I2R heating resulting from the overcurrent. There is a growing interest in providing additional protection, and most importantly arc fault protection. Arc faults are typically high impedance faults and can be intermittent. Nevertheless, such arc faults can result in a fire. During sporadic arcing fault conditions, the overload capability of the circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to activate the automatic trip circuit. An electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker.
Although many circuit breakers also employ ground fault protection, in aircraft applications, the aircraft frame is ground, and there is no neutral conductor. Some aircraft systems have also provided ground fault protection, but through the use of additional devices, namely current transformers which in some cases are remotely located from the protective relay.
In many circuit breaker applications, the voltage across the thermal bimetal element is employed as an indirect measurement of the circuit breaker load current. Sensing current with a bimetal element, as is common practice in circuit breaker based electronics, is complicated by the variation of that element""s impedance as a function of temperature. This variation results in inaccuracies in the measurement of the amplitude of the measured current. For example, the bimetal element""s impedance can vary as much as 70% with temperature over the normal operating range of the circuit breaker depending upon the type of bimetallic material used.
FIG. 1, for example, plots the resistivity (RTY) (ohms circular mil-foot) of a type 6250 bimetal material manufactured by Chace Thermostatic Metals versus temperature (RTemp) (degrees Celsius). As is typical with most metals, the bimetal impedance has a positive temperature coefficient (PTC). In other words, resistance increases with temperature. This variation can cause proportional inaccuracies in the measurement of the load current, thereby affecting performance of control algorithms implemented in the circuit breaker electronics. As bimetal temperature rises, the voltage drop across the bimetal increases for a given amount of load current. The net effect is that the load current appears larger than it really is. Depending on the control algorithm, the potential result could be an errant command to trip the circuit breaker.
FIG. 2 shows an operational amplifier U1 employing thermistor linearization. A linear voltage output, which varies with temperature, is provided by the operational amplifier and a linearized thermistor network including thermistor T1, parallel resistor R1 and series resistor R2. The voltage output decreases linearly as temperature increases. For example, this circuit may be calibrated by adjusting resistor R3 for an output voltage of 200 mV at 25xc2x0 C. and 0 V at 45xc2x0 C.
It is known to employ negative temperature coefficient (NTC) thermistors to compensate for the temperature coefficient response of various components, such as crystal oscillators, mechanical meters and infrared LEDs. FIG. 3 shows a thermistor/resistor network, including thermistor T2, parallel resistor R5 and series resistor R4, placed in parallel with a PTC component METER having a copper coil requiring compensation. The values of the resistors R4 and R5 are selected to provide the proper NTC slope to offset the PTC component.
As shown in FIG. 4, the PTC component response RP and the NTC component response RN combine to provide a net effect in the form of a substantially constant meter circuit response RM that is substantially independent of temperature.
There is room for improvement in circuit breakers and trip assemblies for circuit breakers.
The present invention is directed to a circuit breaker, which compensates for bimetal electrical resistance variation with temperature when, for example, sensing current indirectly from bimetal voltage.
As one aspect of the invention, a circuit breaker comprises: separable contacts; a latchable operating mechanism including a latch member which when released opens the separable contacts; a bimetal in series with the separable contacts and adapted for heating by current flowing therethrough, the bimetal including a temperature, a temperature coefficient, a first terminal, and a second terminal having a voltage, the bimetal being adapted to deflect by the heating, the bimetal coupled to the latch member to release the latch member in response to a persistent overcurrent condition; and a trip assembly comprising: a thermistor adapted to respond to the temperature of the bimetal, an amplifier having a first input, a second input and an output, a first resistor electrically connected between the second terminal of the bimetal and the first input of the amplifier, a second resistor electrically connected in parallel with the thermistor, a third resistor electrically connected in series with the parallel combination of the second resistor and the thermistor, with the series combination of the third resistor and the parallel combination of the second resistor and the thermistor being electrically connected between the first input of the amplifier and the output of the amplifier, the second input of the amplifier being referenced to the first terminal of the bimetal, the output of the amplifier having a voltage which is compensated for the temperature coefficient of the bimetal, means for providing a trip signal as a function of the compensated voltage, and means for releasing the latch member to trip the separable contacts open in response to the trip signal.
The first terminal of the bimetal may have a voltage, the means for providing a trip signal may have a ground which is the voltage of the first terminal of the bimetal, the amplifier may be an operational amplifier having an inverting input as the first input and a non-inverting input as the second input, and a fourth resistor may be electrically connected between the non-inverting input and the ground.
As another aspect of the invention, a method of operating a circuit breaker comprises the steps of: employing a bimetal having a temperature coefficient in series with separable contacts of the circuit breaker; heating the bimetal to a temperature by passing current through the series combination of the bimetal and the separable contacts; employing a thermistor having a first terminal and a second terminal to respond to the temperature of the bimetal; employing an amplifier having a first input, a second input and an output; electrically connecting a first resistor between the second terminal of the bimetal and the first input of the amplifier, electrically connecting a second resistor in parallel with the thermistor; electrically connecting a third resistor in series with the parallel combination of the second resistor and the thermistor; electrically connecting the series combination of the third resistor and the parallel combination of the second resistor and the thermistor between the first input of the amplifier and the output of the amplifier; referencing the second input of the amplifier to the first terminal of the bimetal; outputting a voltage from the output of the amplifier; providing a trip signal as a function of the voltage; and opening the separable contacts of the circuit breaker in response to the trip signal.
As a further aspect of the invention, a trip assembly for a circuit breaker comprises: a bimetal adapted for connection in series with separable contacts of the circuit breaker and adapted for heating by current flowing therethrough, the bimetal including a temperature, a temperature coefficient, a first terminal, and a second terminal having a voltage; a thermistor adapted to respond to the temperature of the bimetal; an amplifier having a first input, a second input and an output; a first resistor electrically connected between the second terminal of the bimetal and the first input of the amplifier; a second resistor electrically connected in parallel with the thermistor; a third resistor electrically connected in series with the parallel combination of the second resistor and the thermistor, with the series combination of the third resistor and the parallel combination of the second resistor and the thermistor being electrically connected between the first input of the amplifier and the output of the amplifier, the second input of the amplifier being referenced to the first terminal of the bimetal, the output of the amplifier having a voltage; and means for providing the trip signal as a function of the voltage of the output of the amplifier.
As another aspect of the invention, a bimetal compensation circuit is for a circuit breaker having separable contacts and a bimetal electrically connected in series with the separable contacts and adapted for heating by current flowing therethrough. The bimetal compensation circuit comprises: a thermistor adapted to respond to a temperature of the bimetal; an amplifier having an input and an output; a first resistor electrically connected between a second terminal of the bimetal and the input of the amplifier; a second resistor electrically connected in parallel with the thermistor; and a third resistor electrically connected in series with the parallel combination of the second resistor and the thermistor, with the series combination of the third resistor and the parallel combination of the second resistor and the thermistor being electrically connected between the input of the amplifier and the output of the amplifier, the output of the amplifier having a voltage, which is compensated for a temperature coefficient of the bimetal.